"And now they're involved in swarm synthesis," I said. "That didn't take long, just a few days. Coevolution in action. The spheres probably provide food, and collect their excretions in some way."

"Or collect them," Mae said dryly.

"Yeah. Maybe." It wasn't inconceivable. Ants raised aphids the way we raised cows. Other insects grew fungus in gardens for food.

We moved deeper into the room. The swarms swirled on all sides of us, but they kept their distance. Probably another unprecedented event, I thought: intruders in the nest. They hadn't decided what to do. I moved carefully; the floor was now increasingly slippery in spots. There was a kind of thick muck on the ground. In a few places it glowed streaky green. The streaks seemed to go inward, toward the center. I had the sense that the floor sloped gently downward. "How much farther?" Mae said. She still sounded calm, but I didn't think she was. I wasn't either; when I looked back I could no longer see the entrance to the chamber, hidden behind the clusters.

And then suddenly we reached the center of the room, because the clusters ended in an open space, and directly ahead I saw what looked like a miniature version of the mound outside. It was a mound about four feet high, perfectly circular, with flat vanes extending outward on all sides. It too was streaked with green. Pale smoke was coming off the vanes. We moved closer.

"It's hot," she said. And it was. The heat was intense; that's why it was smoking. She said, "What do you think is in there?"

I looked at the floor. I could see now that the streaks of green were running from the clusters down to this central mound. I said, "Assemblers." The spiky urchins generated raw organic material. It flowed to the center, where the assemblers churned out the final molecules. This is where the final assembly occurred.

"Then this is the heart," Mae said.

"Yeah. You could say."

The swarms were all around us, hanging back by the clusters. Apparently, they wouldn't come into the center. But they were everywhere around us, waiting for us. "How many you want?" she said quietly, taking the thermite from her pack.

I looked around at all the swarms.

"Five here," I said. "We'll need the rest to get out."

"We can't light five at once…"

"It's all right." I held out my hand. "Give them to me."

"But, Jack…"

"Come on, Mae."

She gave me five capsules. I moved closer and tossed them, unlit, into the central mound. The surrounding swarms buzzed, but still did not approach us.

"Okay," she said. She understood immediately what I was doing. She was already taking out more capsules.

"Now four," I said, looking back at the swarms. They were restless, moving back and forth. I didn't know how long they would stay there. "Three for you, one for me. You do the swarms."

"Right…" She gave me one capsule. I lit the others for her. She threw them back in the direction we had come. The swarms danced away.

She counted: "Three… two… one… now!"

We crouched, ducked away from the harsh blast of light. I heard a cracking sound; when I looked again, some of the clusters were breaking up, falling apart. Spikes were rolling on the ground. Without hesitating, I lit the next capsule, and as it spit white sparks, I tossed it into the central mound.

"Let's go!"

We ran for the entrance. The clusters were crumbling in front of us. Mae leapt easily over the falling spikes, and kept going. I followed her, counting in my mind… three… two… one…

Now.

There was a kind of high-pitched shriek, and then a terrific blast of hot gas, a booming detonation and stabbing pain in my ears. The shock wave knocked me flat on the ground, sent me skidding forward in the sludge. I felt the spikes sticking in my skin all over my body. My goggles were knocked away, and I was surrounded by blackness. Blackness. I could see nothing at all. I wiped the sludge from my face. I tried to get to my feet, slipped and fell. "Mae," I said. "Mae…"

"There was an explosion," she said, in a surprised voice.

"Mae, where are you? I can't see."

Everything was pitch black. I could see nothing at all. I was deep in some damn cave full of spiky things and I couldn't see. I fought panic.

"It's all right," Mae said. In the darkness I felt her hand gripping my arm. Apparently she could see me. She said, "The flashlight's on your belt." She guided my hand. I fumbled in the darkness, feeling for the clip. I found it, but I couldn't get it open. It was a spring clip and my fingers kept slipping off. I began to hear a thrumming sound, low at first, but starting to build. My hands were sweating. Finally the clip opened, and I flicked the flashlight on with a sigh of relief. I saw Mae in the cold halogen beam; she still had her goggles, and looked away. I swung the beam around the cave. It had been transformed by the explosion. Many of the clusters had broken apart and the spikes were all over the floor. Some substance on the floor was beginning to burn. Acrid, foul smoke was billowing up. The air was thick and dark… I stepped backward, and felt something squishy.

I looked down and saw David Brooks's shirt. Then I realized I was standing on what was left of David's torso, which had turned into a kind of whitish jelly. My foot was right in his abdomen. His rib cage scraped against my shin, leaving a white streak on my pants. I looked back and saw David's face, ghostly white and eroded, his features eaten away until he looked as featureless as the faces on the swarms. I felt instant nausea, and tasted bile. "Come on," Mae said, grabbing my arm, squeezing it hard. "Come on, Jack." With a sucking sound, my foot pulled free of the body. I tried to scrape my shoe on the floor, to get clean of the white muck. I was not thinking anymore, I was just fighting nausea and an overpowering sense of horror. I wanted to run. Mae was talking to me but I didn't hear her. I saw only glimpses of the room around me, and was only dimly aware that the swarms were emerging all around us, swarm after swarm after swarm. They were buzzing everywhere. "I need you, Jack," Mae said, holding out four caps, and somehow, fumbling with the flashlight, I managed to light them and she flung them in all directions. I threw my hands over my eyes as the hot spheres exploded around me. When I looked again, the swarms were gone. But in only a few moments, they began to reemerge. First one swarm, then three, then six, then ten-and then too many to count. They were converging, with an angry buzz, toward us. "How many caps have we got left?" I said.

"Eight."

I knew then that we were not going to make it. We were too deep in the cave. We would never get out. I had no idea how many swarms were around us-my halogen beam swung back and forth across what seemed like an army.

"Jack…" Mae said, holding out her hand. She never seemed to lose confidence. I lit three more caps and Mae threw them, retracing her steps toward the entrance as she did so. I stayed close to her, but I knew our situation was hopeless. Each blast scattered the swarms for just a moment. Then they quickly regrouped. There were far too many swarms. "Jack." More thermite in her hands.

Now I could see the entrance to the chamber, just a few yards ahead. My eyes were watering from all the acrid smoke. My halogen light was just a narrow beam cutting through the dust. The air was getting thicker and thicker.

A final series of white-hot blasts, and we came to the entrance. I saw the ramp leading back toward the surface. I never thought we'd get this far. But I wasn't thinking anymore, everything was impressions.

"How many left?" I said.

Mae didn't answer me. I heard the rumble of an engine from somewhere above us. Looking up I saw a wobbling white light in the cave higher up. The rumble became very loud-I heard an engine gunned-and then I saw the ATV poised on the ramp above. Bobby was up there, shouting "Get outttttt!"